This invention relates to a miniature electrical switching unit, and more particularly to a miniature switching unit having a long service life whose contact piece does not yield due to prolonged use.
A switching unit is often used for switching over electric circuits. For example, as shown in FIG. 8, a switching unit 4 is used for switching over the internal and external antennas 1 and 2 of a transmitter-receiver 3 to switch over transmission and reception. FIG. 9 illustrates a switching unit hitherto used in such a case, which includes one terminal 6 having a spring contact piece 5 fixed to its end and the other terminal 8 having at its end a contact 7. These terminals 6 and 8 are fixed in the housing 9 so that one end of the spring contact piece 5 is always in contact with the contact 7 of the terminal 8 under a pressure to connect the internal antenna 1 and the transmitter-receiver 3.
On the other hand, the housing 9 is integrally formed with a cylindrical support portion 9a in which a connection unit 12 is provided. The connection unit 12 includes a movable contact element 11 integrally formed at its upper end with a flange 11a, and a return spring 10 having an upper end abutting against the flange 11a of the movable contact element 11 to urge the contact element 11 upwardly. When a mating connection unit 13 connected with the external antenna 2 and having a fixed contact element 14 is inserted into and connected to the connection unit 12, the movable contact element 11 is moved downwardly by the fixed contact element 14 of the mating connection unit 13 against the spring force of the spring contact piece 5. As a result, the spring contact piece 5 comes out of contact with the contact 7 of the terminal 8 as shown in broken lines in FIG. 9 so that the external antenna 2 is connected to the transmitter-receiver 3.
With recent miniaturization of communication appliances, switching units for use in wireless appliances, for example, portable wireless appliances have been strongly required to be more miniaturized. As a result, there have been very small switching units, for example, having one side length less than 15 mm. Consequently, the spring contact piece 5 has become very short, less than 10 mm, which is repeatedly deformed correspondingly by the vertical movements of the movable contact element 11. Accordingly, the spring contact piece 5 encounters the following problems in design and manufacture:
The shorter the spring contact piece 5, the greater is the moving distance of the contact piece 5 relative to its length by the movable contact element 11. Therefore, the design of the spring contact piece 5 becomes difficult due to its elastic limit. Even if the design is possible, produced spring contact pieces 5 are poor in uniformity and hence exhibit spring forces in a wide range resulting in different contact pressures with contacts 7 of terminals 8 with less reliability in connection. In addition thereto, the spring contact piece 5 is likely to be fatigued or to yield in a short period of time and thus shorten its service life. Moreover, when the spring contact piece 5 is returned to its original position after release from the movable contact element 11, the spring contact piece 5 contacts the contact 7 of the terminal 8 only with low pressure due to its yielding, which tends to cause insufficient connection with the contact 7. Therefore, reliable connection as a switching unit cannot be accomplished.